The invention relates to a layer arrangement, especially for implementation in transfer films or laminated films, which exhibits at least two superposed material layers, of which at least that (or those) facing the observer in use has or have a transparent or semi-transparent appearance, and between which an interface is formed which exhibits, in at least one area thereof, a diffractive optical structure producing some lens-like effect, either magnifying or de-magnifying.
In this context, transfer films include especially so-called embossing films, which consist of a base film and a transfer layer that is detachable from the base film for transference to a substrate. Usually the transfer layer of embossing films is composed of lacquer layers, which means that, in the present invention, the term “material layer” principally means lacquer layer, and at times also adhesive layer. However, the invention also encompasses other embodiments, in which a “material layer” is formed by ambient air or a metallic, dielectric, or semiconductor coating. The structure of laminated films coincides substantially with that of transfer films, with the exception however that the synthetic resin layers or lacquer layers are not detachable from the base film, but rather can be affixed together with the base film to a substrate. Transfer films and laminated films with layer arrangements of this sort are in particular used for security purposes, although they are also used in decorative applications.
Layer arrangements of the type mentioned above are currently known and are coming into use, for example, in the form of a lens having a uniform appearance and used as security device in credit cards (Amex-Blue) new on the market. In these credit cards, the lens-like effect is manifested over an area of comparatively large diameter, and has substantially the form of a circular lens. In the lens-like effect produced by diffractive optical structures of known layer arrangements, a structure produced by a holographic technique is used, which in general possesses a sinusoidal surface profile. Such holographically manufactured lenses have many shortcomings, quite apart from the fact that the holographic manufacture of diffractive optical structures with lens effects, with comparably small technical effort, is only possible when lenses having circular or, at best, elliptical shapes are involved. One drawback of holographically produced lenses is, for example, that they are not very bright in appearance and in general they exhibit irregularities, especially in the central area, whereby the visual effect that the lens should produce is considerably degraded. A further disadvantage of holographically produced lenses is that it is virtually impossible to achieve certain color effects with any great freedom of design.